1. Field of the Invention
The present invention relates to a magnetic recording tape and more particularly, it is concerned with an improved magnetic recording tape having a small total thickness but having an excellent recording and reproducing property as well as a practical durability.
2. Description of the Prior Art
In order to raise the packing density of a magnetic recording tape, it has hitherto been proposed to reduce the total thickness of the tape and, for example, Japanese Patent Publication No. 4623/1974 discloses a Philips type cassette tape, whereby the practical durability can be improved. That is to say, magnetic recording tapes have the disadvantage that, if the thickness is decreased, the recording and reproducing property is deteriorated even by a slight external force since the tape tends to be deformed, but, in the Philips type cassette tape, this disadvantage can be overcome by packing the magnetic recording tape into a magazine so that it be used without touching. However, even the Philips type cassette tape has also the disadvantage that the tape speed is markedly uneven even during ordinary tape running and a tape vibration is induced at a position where the tape is pressed to a guide roller or head, when the total thickness of the tape is 14.5 microns or less and the tensile strength at an elongation of 0.5% (which will hereinafter be referred to as "F-0.5 Value") is about 0.13 d (Kg) to the total thickness d (.mu.). Furthermore, in some cases, a magnetic recording tape with a small total thickness encounters a large winding disorder and in extreme cases, the end of the tape is broken so that it be of no use.
In the Philips type cassette tape of the prior art, where the tape has a total thickness of 13.+-.1.5.mu. (C 90 Tape according to Jiki Tape Kogyokai Standard MTS-101) or where the tape has a magnetic layer with a thickness of about 4 and a total thickness of 9.+-.1.5.mu.(C 120 Tape), it has been employed to make thinner the thickness of a base and that of a magnetic layer proportionally based on that the thickness of the magnetic layer is about 3.mu. and the proportion of the thickness of the magnetic layer to the total thickness is about 33%, so as to reinforce the above described mechanical strength. When magnetic recording tapes with further increased packing densities, for example, C 180 Tape having a mean thickness of 6.mu. and C 240 Tape having a mean thickness of 4.5.mu. are prepared, it is necessary to adjust the total thickness to 7.5.mu. or less and to 5.5.mu. or less respectively. Furthermore, when a magnetic layer is provided by blending and coating a magnetic powder and binder, it is not desirable to make thinner proportionally the thickness of the magnetic layer from the standpoint of its magnetic property. When a magnetic recording tape with a decreased thickness of magnetic layer is used in the same recording and reproducing system as used before decreasing the thickness of magnetic layer, it exhibits a mortal disadvantage that the harmonic distortion factor at a constant output is increased and, consequently, the dynamic range representing a range of from the noise level to the output level at 5% third harmonic distortion as to a signal to be recorded is narrowed.